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Capx Chain Consensus
(L2: Capx Chain Execution & Proposal)

  1. User ---sends L2 Tx---> [Capx Chain Sequencer]
  2. [Sequencer]
    • Orders transactions.
    • Executes transactions, computes new L2 state.
    • Creates a [Transaction Batch] and a new [L2 State Root_S].
    • Provides ---Soft Confirmation---> User.
  3. [Sequencer] ---submits Tx Batch (calldata)---> [L1 Ethereum: Inbox Contract]
    • Data is now anchored on L1, ensuring Data Availability.

(L1: Assertion & Challenge Period)

  1. [Asserter (e.g., Sequencer)] ---posts Assertion(L2 State Root_S)---> [L1 Ethereum: Rollup Core Contract]
  2. [Rollup Core Contract] ---initiates Challenge Period (e.g., 7 days)---
    • During this period, any validator can challenge the assertion.
    • Validators can independently verify the state root by executing the transactions in the batch.
    • The challenge period allows for a dispute resolution process if discrepancies arise.

(L2 & L1: Validation & Potential Challenge)

  1. [Capx Chain Validators (L2 Full Nodes)]:
    • Fetch [Transaction Batch] from L1 Inbox.
    • Independently execute transactions, compute their own [L2 State Root_V].
    • Compare [L2 State Root_V] with asserted [L2 State Root_S] on L1.
  2. Decision Point during Challenge Period:
    • IF State Root_V == State Root_S (Happy Path):
      • Validators take no action against this assertion.
      • IF Challenge Period Ends without successful challenge:
        • [L1 Ethereum: Rollup Core Contract] ---confirms Assertion---> [L2 State Root_S is Finalized] ✅
        • This is L1-Secured Finality for the L2 Block.
    • IF State Root_V != State Root_S (Dispute Path):
      • [A Challenging Validator] ---initiates Fraud Proof against Assertion(L2 State Root_S)---> [L1 Ethereum: Rollup Core Contract]
      • Fraud Proof Process (Interactive Bisection in Nitro):
        • The contract facilitates an interactive game between the Asserter and Challenger.
        • They narrow down the dispute to a single differing instruction in the L2 execution.
        • This single instruction is then executed via a WASM module on L1 (the OneStepProof mechanism) to determine the correct state transition.
        • IF Challenger Wins: Fraudulent assertion is rejected, Challenger is rewarded, Asserter is penalized. The correct state can be asserted.
        • IF Asserter Wins: Challenge is dismissed, Challenger may lose their stake. Assertion proceeds towards finalization.

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Key Components & Roles in Consensus:

  1. Sequencer (L2):
    • Role: A designated node (or a decentralized set of nodes in future iterations of Arbitrum technology) responsible for accepting user transactions, determining their order, executing them, and bundling them into compressed batches.
    • Block Proposal (L2): The Sequencer effectively proposes L2 “blocks” or batches by creating an ordered sequence of transactions and calculating the resulting state transition. This provides users with fast, sub-second “soft” confirmations or pre-confirmations.
    • L1 Interaction: Submits these transaction batches as calldata to an Inbox contract on Ethereum L1. This ensures data availability.
  2. Asserters (Typically includes the Sequencer, can be other permissioned L2 Nodes):
    • Role: After L2 blocks are processed and data is posted to L1, an Asserter posts a cryptographic commitment (a state root) of Capx Chain’s new state to the Rollup Core contract on Ethereum L1. This is an “assertion” about the outcome of executing a specific batch of L2 transactions.
    • State Finalization (Path to): These assertions initiate a challenge period on L1.
  3. Validators (Full L2 Nodes):
    • Role: These nodes independently execute all L2 transactions from the data available on the L1 Inbox. They compute their own version of the L2 state root.
    • Verification: They compare their calculated state root against the state root asserted on L1 by the Asserter.
    • Challenge Initiation: If a Validator detects a discrepancy (i.e., the asserted state root is incorrect according to their computation), they can initiate a fraud proof challenge on L1 during the challenge period.
  4. Ethereum L1 Contracts (e.g., Rollup Core, Inbox, Bridge):
    • Role: These smart contracts on Ethereum are the arbiters of truth for Capx Chain.
      • Inbox: Receives transaction data from the Sequencer.
      • Rollup Core: Manages the state of the rollup, tracks assertions, and facilitates the fraud proof mechanism.
    • Finality: L1 provides the ultimate settlement and finality. An L2 state root is considered final once its assertion has survived the challenge period on L1, or a fraud proof has successfully defended it (or corrected a fraudulent one).
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